How to Grow Your Own Koji

Koji is the heart of sake. One of the most popular phrases for brewers is “Ichi: koji. Ni: moto. San: tsukuri” or first the koji, then the yeast starter, then the main mash. It serves a multitude of purposes: converting starch to sugar, converting protein into amino acids (umami), and creating all sorts of other compounds for yeast health and flavor.

Aside from sake, it is also used to make miso, soy sauce, mirin, and many other culinary products. There are tons of differences between koji rice used for sake and for miso, but the main method remains the same.

Here is a quick, simple guide to growing your own:

1) Wash your rice and then soak it in room temperature water for 8-12 hours. The time depends on the type of rice, but you are aiming for about 30-32% moisture gain. You can measure this by weighing the rice before and after

2) Drain the rice and prepare a steamer. Steam the rice for 60 minutes, making sure not to overfill the baskets and that they steam evenly.

3) Let the rice cool to about 90*F.

4) Sprinkle koji spores on the rice. These can be procured from GEM Cultures if you’re in the US. The typical ratio is 1g of spores to 1kg of cooked rice. To make it easier, you can disperse the spores in rice flour to get a more accurate measurement and distribution

5) Place the inoculated rice in an environment that is around 90% humidity and 86*F. I place a baking pan raised above water in a hotel pan. I then use a sous vide circulator to keep the temperature.

6) Every 12 hours, gently break up any clumps of rice and mix it to distribute moisture and temperature.

7) After 48 hours, you should see heavy growth on the grains of rice. It may take longer. If you begin to see any green coloring, that is the koji making spores, and you’ve gone too far.

8) You can then store your koji in the refrigerator if you’re using within a week, or the freezer to keep it for longer.